Low voltage circuit breaker is an important electrical equipment for electricity transmission and distribution and plays an important role in distributing electric power and protecting devices and personnel safety in electrical circuits. Under normal working conditions, a low voltage circuit breaker distributes and controls the power consumption load by connecting or disconnecting its contactors. When a malfunction occurs in the electrical circuit, the circuit breaker detects a malfunction signal and actuates a disconnector to separate the contactors, thereby disconnecting the faulty circuit line. If a short-circuit malfunction occurs in the electrical circuit, when disconnecting the short-circuit current, quick extinguishing of electric arc is an important condition to ensure reliable disconnection of the circuit breaker. In an alternating current system, because the alternating current has zero crossing points, it is relatively easy to extinguish an electric arc. However, as direct current does not have a zero crossing point, it is relatively difficult to extinguish an electric arc. As present, a popular method is to install a permanent magnet to enhance the arc extinguishing effect. Although this kind of method helps to extinguish an electric arc, it limits the wiring manner of positive and negative electrodes of the circuit breaker.
Chinese patent literature CN204067289U disclosed a direct current circuit breaker with plastic casing, which comprises a first casing and a second casing that are detachably connected, wherein, several evenly distributed arc extinguishing grid-plates are provided inside an arc extinguishing chamber, the arc extinguishing chamber has an upper end opening that faces the first casing, a gas generating bracket corresponding to the arc extinguishing grid-plates is provided inside the arc extinguishing chamber, with one end of the gas generating bracket disposed between the first casing and the upper end opening and arranged to cover the upper end opening; the gas generating bracket has two oppositely disposed gas generating arms with one end thereof provided with a shield plate, the shield plate is disposed between the first casing and the upper end opening and arranged to cover the upper end opening.
In the direct current circuit breaker with plastic casing of this patent literature, although no permanent magnet is installed to enhance the arc extinguishing effect and thus the wiring manner of positive and negative electrodes of the circuit breaker is not limited, a shield plate is added between the upper end opening of the arc extinguishing chamber and the first casing, such design may prevent the pressurized gas produced by electric arc combustion from leaking out at this location and thus protect the first casing from being impinged upon and cracking, however, there is the following problem: as seen from FIG. 6 of its Specification, the disconnecting portion between a movable contactor and a stationary contactor is near the second casing and far from the shield plate, therefore, when the movable contactor becomes disconnected from the stationary contactor and an electric arc is produced, the gas generating arm closest to the electric arc is heated and generates gas, so as to push the electric arc to move towards the shield plate; during the moving process of the electric arc towards the shield plate, because of a shielding effect of the shield plate, the gas pressure increases rapidly, which blocks the motion of the electric arc and causes the moving distance thereof to be short, and under the relatively larger gas pressure, the electric arc moves towards the arc extinguishing grid-plates when moving upwards; because the moving distance of the electric arc is short, the upward moving electric arc can only contact the part of the arc extinguishing grid-plates that are close to the disconnecting portion between the movable contactor and the stationary contactor, and is unable to fully utilize a majority of the arc extinguishing grid-plates, which causes the temperature of the part of the arc extinguishing grid-plates close to the disconnecting portion to rapidly become high, not good for quick cooling of the electric arc and thus not good for quick extinguishing of the electric arc, and if the electric arc is still not extinguished after passing through the arc extinguishing grid-plates, danger might be incurred.